As is well known, a standardization for high-efficiency encoding techniques of digital data has been enthusiastically promoted. At present there are three standards, i.e., (1) CCITT Recommendation H261 for video conference/video telephone., (2) JPEG (Joint Photograghic Expert Group) for still color pictures; and (3) MPEG (Moving Picture Expert Group) for a storage media such as a CD-ROM (Compact Disc Read Only Memory) (see NIKKEI ELECTRONICS, Oct. 15, 1990, No. 511, pp.124-129). On the other hand, a digital recording/reproducing apparatus is also being developed which records information signals on a recording medium in a highly efficient encoded form and decodes the high-efficiency encoded information signals read from the recording medium to thereby reproduce original information signals. In the digital recording apparatus, video cassette recorders (VCRs), using magnetic tapes as a recording medium, and IC (Integrated Circuit) memory devices), using semiconductor memories as a recording medium, have been developed.
At present a leading high-efficiency encoding technique uses an orthogonal transform technique which is referred to as a DCT (Discrete Cosine Transform) technique.
FIG. 1 illustrates a conventional recording apparatus utilizing such a DCT technique. In FIG. 1, a data input terminal 11 is supplied with a digital data such as digital image data as a valid transmission information signal. The digital image data is transmitted on a field-sequential basis in the NTSC system. Thus, two successive fields stored in frame memory 12 produce one frame of the digital image data. In this case, the digital image data is produced by encoding a luminance signal Y, a red signal component Cr and a blue signal component Cb. Unless otherwise specified, the luminance signal Y is taken as an example in the following description.
When one frame of the digital image data is sampled at, for example, a frequency of 4fsc (14.3 MHz), 910 pixels are present on one horizontal line, because there are 910 samples in the horizontal direction. There are 525 lines in the vertical direction and thus 525 pixels are present in the vertical direction. That is, there are a total of 910 .times. 525 pixels present at the time of sampling. However, only about 80% of the total number of pixels (768 pixels in the horizontal direction .times. 488 pixels in the vertical direction) can be visible on the screen as valid pixels. These valid pixels constitute digital image data applied to the input terminal as a valid transmission information signal. One frame of digital image data stored in the frame memory 12 is read in blocks of four pixels (horizontal) x four pixels (vertical) and applied to a DCT unit 13 where the digital data is subjected to an orthogonal transform process on a block-by-block basis.
The orthogonal transform converts the axis of digital image data from the time axis to the frequency axis on a block basis. The orthogonal transform also converts low frequency components into high frequency components in the order of increasing frequency, two-dimensionally, in both the horizontal and the vertical directions. Thus, the digital image data is arranged such that it changes from a direct current through a low frequency to a high frequency in a zigzag scanning fashion as indicated FIG. 2 by the arrow which advances in the horizontal and the vertical directions. The data subjected to the orthogonal transform is delayed by a frame delay unit I4 by one-frame period of time corresponding to a time calculated by an activity calculator 22 as described later and then applied to a scanning converter 15.
The scanning converter 15 scans data in every block in a zigzag manner, as indicated by the arrow in FIG. 2, on the basis of the contents in a standard scanning table 16 stored in a suitable memory. Furthermore it rearranges them one-dimensionally so that the DC (direct current) components to high frequency components can be output in sequence in the order of increasing frequency in the horizontal and the vertical directions. This is because, from the standpoint of reproduction of an original image, when the bit rate is decreased, sequential transmission of DC components to high frequency components in the order of increasing frequency can reproduce a visually good image at a lower bit rate and, hence, with higher efficiency, The data scanned and converted in such a manner has a generally larger data amount than the original digital image data. Thus, data compression is not achieved without modification. For this reason, a quantizer 17 is used for requantization.
The quantizer 17 reduces an amount of data from the scanning converter 15 by dividing the data by corresponding contents in a multiplied quantization table, i.e., the result of multiplication of the contents of a basic quantization table 18 stored in a Suitable memory and a suitable coefficient A, which will be described later, by a multiplier 19. The data requantized by the quantizer 17 is further applied to a variable length encoder 20 for high-efficient transmission encoding. The encoding technique, which is used most by the variable-length encoder 20 is the HUffman encoding or Run Length encoding, in which the number of successive "0"s and the number of digits other than "0" following the "0" in a requantized output are combined to allocate fewer bits in the order of decreasing probability of its occurrence. The number of bits is two at a minimum and several tens at a maximum. Thus, the data compression is performed on the digital image data. The data subjected to the data compression is provided for digital recording on a recording medium (not shown) through a data output terminal 21.
In order to compress data while maintaining picture quality, the requantizing process by the quantizer 17 is the most Important. The performance of the requantization depends on the calculation of coefficient A, by which the basic quantization table 18 is multiplied, according to the basic quantization table 18 and the input digital image data. The picture definition (a rate at which fine detail and high frequency components are contained in a picture) is used for the calculation. That is, the coefficient A is calculated by the activity calculator 22 using, as a measure of evaluation, a normal deviation or a quantity extracted from high frequency components output from the DCT unit 13. The result of this calculation is converted to the coefficient A by a coefficient converter 23, which is in turn applied to a multiplier 19.
At the time of reproducing of the high-efficiency encoded data from the recording medium, the variable-length encoded data is read from the recording medium and then subjected to processes which are the inverse of those at the time of the encoding of data, i.e., inverse quantization, inverse scanning conversion and inverse DCT processing. Thereby, the original digital image data is recovered and displayed as an picture.
Now the requantizing operation of the quantizer 17 will be described. Suppose that such a scanning table as shown in FIG. 3A is applied from the scanning converter 15 to the quantizer 17, while an operative quantization table, which is a multiplication of the basic quantization table as shown in FIG. 3B, stored the basic quantization table 18 with the coefficient "2" supplied from the coefficient converter 23. Then, the quantizer 17, which multiplies each of the values of the basic quantization table 18 by the coefficient "2" and divides a corresponding input value of the quantizer 17 by each of the multiplication results As shown in FIG. 3C, therefore, each output of the quantizer 17 is decreased in quantity of data. The data compressed as shown in FIG. 3C is subjected to the variable-length encoding process and then recoded on the first recording medium.
At the time of reproducing of the first recording medium, an expansion of data is performed by the inverse quantization processing by multiplying each input value of the basic quantization table 18 by the coefficient "2" used at the time of the compression and multiplying each value compressed as shown in FIG. 3C by a corresponding one of the multiplication results. In this case, as shown in FIG. 3D, the results of the inverse quantization the portions which are marked with * have values smaller than the corresponding original values shown in FIG. 3A. That is, data deterioration occurs in that portion. However, with the high-efficiency encoding process using the DCT technique, such a degree of data deterioration is inevitable memory because it is an irreversible encoding system and is not a problem to be solved by the present invention (at this time, a truncating process is used for the calculation, discarding the decimal fractions).
In consideration of the conventional recording apparatus having a copy prohibiting function, a microcomputer controls the recording apparatus to detect copy prohibiting information from the input digital data and to ignore any operation for recording keys so that circuits associated for the recording operation is deactivated. However, such a copy prohibiting operation has a problem to mislead an operator to misunderstand as if the recording apparatus were caught in any trouble, because of the deactivation of the recording operation.